The present invention relates to a compound semiconductor device based on gallium nitride, which is suitable for a light emitting device such as a light emitting diode, a laser diode or the like.
A compound semiconductor based on indium gallium nitride (InxGa1xe2x88x92xN) has been paid attention as a material that can obtain light emission having wavelength in the visible range by varying a composition (x) of indium. A ridge-shaped MQW laser diode based on InGaN using such material has been proposed (see S17 to S20 in a script for the 17th annual lecture of The Laser Society of Japan).
Above mentioned ridge-shaped MQW laser diode based on InGaN, as shown in FIG. 12, is made by forming a GaN buffer layer 42, an n-type GaN contact layer 43, an n-type InGaN crack-preventive layer 44, an n-type AlyGa1xe2x88x92yN clad layer 45, an n-type GaN guide layer 46, an MQW active layer composed of InGaN 47, a p-type AlGaN cap layer 48, a p-type GaN guide layer 49, a p-type AlyGa1xe2x88x92yN clad layer 50, and a p-type GaN contact layer 51 in this order on a sapphire substrate 41. A p-electrode 52 is formed on the p-type contact layer 51 and an n-electrode 53 is formed after etching to the n-type contact layer 43.
In the compound semiconductor device based on indium gallium nitride such as the ridge-shaped MQW laser diode based on InGaN, a crack is generated on a clad layer due to a difference in lattice constant or coefficient of thermal expansion when an AlGaN, as the clad layer, is formed on a contact layer by epitaxial growth. As a result, a thick AlGaN layer can not be formed. Although the n-type InGaN crack-preventive layer is interposed between the clad layer and the contact layer in the ridge-shaped MQW laser diode based on InGaN, the clad layer containing AlGaN to be formed on the n-type InGaN crack-preventive layer cannot be thick because the clad layer containing AlGaN is different from the n-type InGaN crack-preventive layer in lattice constant or coefficient of thermal expansion.
The present invention was made in consideration of the above mentioned present problems and has an objective to provide a semiconductor device capable of preventing cracks or defects attributable to a strain caused by a difference in lattice constant and coefficient of thermal expansion and forming a thick gallium nitride semiconductor layer.
A compound semiconductor device based on gallium nitride according to the present invention comprises a buffer layer having both compositions substantially the same as those of a first compound semiconductor layer based on gallium nitride and a second compound semiconductor layer based on gallium nitride is interposed between the first compound semiconductor layer based on gallium nitride and the second compound semiconductor layer based on gallium nitride of which a composition is different from that of the first compound semiconductor layer based on gallium nitride.
The buffer layer has a superstructure layer formed by alternately laminating a layer which has substantially the same composition as that of the first compound semiconductor layer based on gallium nitride and a layer which has substantially the same composition as that of the second compound semiconductor layer based on gallium nitride.
Furthermore, the buffer layer can be formed by varying a composition ratio from a composition ration substantially the same as that of the first compound semiconductor layer based on gallium nitride to a composition ratio substantially the same as that of the second compound semiconductor layer based on gallium nitride.
A compound semiconductor device based on gallium nitride according to the present invention comprises an one-type conductive contact layer containing a first compound semiconductor layer based on gallium nitride, an one-type conductive clad layer containing a second compound semiconductor layer based on gallium nitride having a different composition from the first compound semiconductor layer based on gallium nitride, an active layer of a single quantum well structure composed of indium gallium nitride, an other-type conductive clad layer containing a second compound semiconductor layer based on gallium nitride having a different composition from the first compound semiconductor layer based on gallium nitride, and an other-type conductive contact layer containing the first compound semiconductor layer based on gallium nitride, and a buffer layer having both compositions substantially the same as those of the first and second compound semiconductor layers based on gallium nitride are interposed at least between the one-type conductive contact layer and the other-type conductive clad layer.
The buffer layer having both compositions substantially the same as those of the first and second compound semiconductor layers based on nitride gallium is interposed above and below the clad layer.
When the buffer layer is formed between the first and second compound semiconductor layers based on gallium nitride, the buffer layer modifies a difference in lattice constant and coefficient of thermal expansion and prevents cracks and defects caused by a difference in lattice constant or coefficient of thermal. Accordingly, even when eptaxially growing a thick n-type or p-type compound semiconductor layer based on gallium nitride, a crack does not occur, thus realizing a light emitting element capable of effective carrier confinement and optical confinement in an active layer.